xref: /openbmc/linux/mm/memfd.c (revision e620a1e0)
1 /*
2  * memfd_create system call and file sealing support
3  *
4  * Code was originally included in shmem.c, and broken out to facilitate
5  * use by hugetlbfs as well as tmpfs.
6  *
7  * This file is released under the GPL.
8  */
9 
10 #include <linux/fs.h>
11 #include <linux/vfs.h>
12 #include <linux/pagemap.h>
13 #include <linux/file.h>
14 #include <linux/mm.h>
15 #include <linux/sched/signal.h>
16 #include <linux/khugepaged.h>
17 #include <linux/syscalls.h>
18 #include <linux/hugetlb.h>
19 #include <linux/shmem_fs.h>
20 #include <linux/memfd.h>
21 #include <uapi/linux/memfd.h>
22 
23 /*
24  * We need a tag: a new tag would expand every xa_node by 8 bytes,
25  * so reuse a tag which we firmly believe is never set or cleared on tmpfs
26  * or hugetlbfs because they are memory only filesystems.
27  */
28 #define MEMFD_TAG_PINNED        PAGECACHE_TAG_TOWRITE
29 #define LAST_SCAN               4       /* about 150ms max */
30 
31 static void memfd_tag_pins(struct xa_state *xas)
32 {
33 	struct page *page;
34 	unsigned int tagged = 0;
35 
36 	lru_add_drain();
37 
38 	xas_lock_irq(xas);
39 	xas_for_each(xas, page, ULONG_MAX) {
40 		if (xa_is_value(page))
41 			continue;
42 		page = find_subpage(page, xas->xa_index);
43 		if (page_count(page) - page_mapcount(page) > 1)
44 			xas_set_mark(xas, MEMFD_TAG_PINNED);
45 
46 		if (++tagged % XA_CHECK_SCHED)
47 			continue;
48 
49 		xas_pause(xas);
50 		xas_unlock_irq(xas);
51 		cond_resched();
52 		xas_lock_irq(xas);
53 	}
54 	xas_unlock_irq(xas);
55 }
56 
57 /*
58  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
59  * via get_user_pages(), drivers might have some pending I/O without any active
60  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
61  * and see whether it has an elevated ref-count. If so, we tag them and wait for
62  * them to be dropped.
63  * The caller must guarantee that no new user will acquire writable references
64  * to those pages to avoid races.
65  */
66 static int memfd_wait_for_pins(struct address_space *mapping)
67 {
68 	XA_STATE(xas, &mapping->i_pages, 0);
69 	struct page *page;
70 	int error, scan;
71 
72 	memfd_tag_pins(&xas);
73 
74 	error = 0;
75 	for (scan = 0; scan <= LAST_SCAN; scan++) {
76 		unsigned int tagged = 0;
77 
78 		if (!xas_marked(&xas, MEMFD_TAG_PINNED))
79 			break;
80 
81 		if (!scan)
82 			lru_add_drain_all();
83 		else if (schedule_timeout_killable((HZ << scan) / 200))
84 			scan = LAST_SCAN;
85 
86 		xas_set(&xas, 0);
87 		xas_lock_irq(&xas);
88 		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
89 			bool clear = true;
90 			if (xa_is_value(page))
91 				continue;
92 			page = find_subpage(page, xas.xa_index);
93 			if (page_count(page) - page_mapcount(page) != 1) {
94 				/*
95 				 * On the last scan, we clean up all those tags
96 				 * we inserted; but make a note that we still
97 				 * found pages pinned.
98 				 */
99 				if (scan == LAST_SCAN)
100 					error = -EBUSY;
101 				else
102 					clear = false;
103 			}
104 			if (clear)
105 				xas_clear_mark(&xas, MEMFD_TAG_PINNED);
106 			if (++tagged % XA_CHECK_SCHED)
107 				continue;
108 
109 			xas_pause(&xas);
110 			xas_unlock_irq(&xas);
111 			cond_resched();
112 			xas_lock_irq(&xas);
113 		}
114 		xas_unlock_irq(&xas);
115 	}
116 
117 	return error;
118 }
119 
120 static unsigned int *memfd_file_seals_ptr(struct file *file)
121 {
122 	if (shmem_file(file))
123 		return &SHMEM_I(file_inode(file))->seals;
124 
125 #ifdef CONFIG_HUGETLBFS
126 	if (is_file_hugepages(file))
127 		return &HUGETLBFS_I(file_inode(file))->seals;
128 #endif
129 
130 	return NULL;
131 }
132 
133 #define F_ALL_SEALS (F_SEAL_SEAL | \
134 		     F_SEAL_SHRINK | \
135 		     F_SEAL_GROW | \
136 		     F_SEAL_WRITE | \
137 		     F_SEAL_FUTURE_WRITE)
138 
139 static int memfd_add_seals(struct file *file, unsigned int seals)
140 {
141 	struct inode *inode = file_inode(file);
142 	unsigned int *file_seals;
143 	int error;
144 
145 	/*
146 	 * SEALING
147 	 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
148 	 * but restrict access to a specific subset of file operations. Seals
149 	 * can only be added, but never removed. This way, mutually untrusted
150 	 * parties can share common memory regions with a well-defined policy.
151 	 * A malicious peer can thus never perform unwanted operations on a
152 	 * shared object.
153 	 *
154 	 * Seals are only supported on special tmpfs or hugetlbfs files and
155 	 * always affect the whole underlying inode. Once a seal is set, it
156 	 * may prevent some kinds of access to the file. Currently, the
157 	 * following seals are defined:
158 	 *   SEAL_SEAL: Prevent further seals from being set on this file
159 	 *   SEAL_SHRINK: Prevent the file from shrinking
160 	 *   SEAL_GROW: Prevent the file from growing
161 	 *   SEAL_WRITE: Prevent write access to the file
162 	 *
163 	 * As we don't require any trust relationship between two parties, we
164 	 * must prevent seals from being removed. Therefore, sealing a file
165 	 * only adds a given set of seals to the file, it never touches
166 	 * existing seals. Furthermore, the "setting seals"-operation can be
167 	 * sealed itself, which basically prevents any further seal from being
168 	 * added.
169 	 *
170 	 * Semantics of sealing are only defined on volatile files. Only
171 	 * anonymous tmpfs and hugetlbfs files support sealing. More
172 	 * importantly, seals are never written to disk. Therefore, there's
173 	 * no plan to support it on other file types.
174 	 */
175 
176 	if (!(file->f_mode & FMODE_WRITE))
177 		return -EPERM;
178 	if (seals & ~(unsigned int)F_ALL_SEALS)
179 		return -EINVAL;
180 
181 	inode_lock(inode);
182 
183 	file_seals = memfd_file_seals_ptr(file);
184 	if (!file_seals) {
185 		error = -EINVAL;
186 		goto unlock;
187 	}
188 
189 	if (*file_seals & F_SEAL_SEAL) {
190 		error = -EPERM;
191 		goto unlock;
192 	}
193 
194 	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
195 		error = mapping_deny_writable(file->f_mapping);
196 		if (error)
197 			goto unlock;
198 
199 		error = memfd_wait_for_pins(file->f_mapping);
200 		if (error) {
201 			mapping_allow_writable(file->f_mapping);
202 			goto unlock;
203 		}
204 	}
205 
206 	*file_seals |= seals;
207 	error = 0;
208 
209 unlock:
210 	inode_unlock(inode);
211 	return error;
212 }
213 
214 static int memfd_get_seals(struct file *file)
215 {
216 	unsigned int *seals = memfd_file_seals_ptr(file);
217 
218 	return seals ? *seals : -EINVAL;
219 }
220 
221 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
222 {
223 	long error;
224 
225 	switch (cmd) {
226 	case F_ADD_SEALS:
227 		/* disallow upper 32bit */
228 		if (arg > UINT_MAX)
229 			return -EINVAL;
230 
231 		error = memfd_add_seals(file, arg);
232 		break;
233 	case F_GET_SEALS:
234 		error = memfd_get_seals(file);
235 		break;
236 	default:
237 		error = -EINVAL;
238 		break;
239 	}
240 
241 	return error;
242 }
243 
244 #define MFD_NAME_PREFIX "memfd:"
245 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
246 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
247 
248 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
249 
250 SYSCALL_DEFINE2(memfd_create,
251 		const char __user *, uname,
252 		unsigned int, flags)
253 {
254 	unsigned int *file_seals;
255 	struct file *file;
256 	int fd, error;
257 	char *name;
258 	long len;
259 
260 	if (!(flags & MFD_HUGETLB)) {
261 		if (flags & ~(unsigned int)MFD_ALL_FLAGS)
262 			return -EINVAL;
263 	} else {
264 		/* Allow huge page size encoding in flags. */
265 		if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
266 				(MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
267 			return -EINVAL;
268 	}
269 
270 	/* length includes terminating zero */
271 	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
272 	if (len <= 0)
273 		return -EFAULT;
274 	if (len > MFD_NAME_MAX_LEN + 1)
275 		return -EINVAL;
276 
277 	name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
278 	if (!name)
279 		return -ENOMEM;
280 
281 	strcpy(name, MFD_NAME_PREFIX);
282 	if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
283 		error = -EFAULT;
284 		goto err_name;
285 	}
286 
287 	/* terminating-zero may have changed after strnlen_user() returned */
288 	if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
289 		error = -EFAULT;
290 		goto err_name;
291 	}
292 
293 	fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
294 	if (fd < 0) {
295 		error = fd;
296 		goto err_name;
297 	}
298 
299 	if (flags & MFD_HUGETLB) {
300 		struct user_struct *user = NULL;
301 
302 		file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
303 					HUGETLB_ANONHUGE_INODE,
304 					(flags >> MFD_HUGE_SHIFT) &
305 					MFD_HUGE_MASK);
306 	} else
307 		file = shmem_file_setup(name, 0, VM_NORESERVE);
308 	if (IS_ERR(file)) {
309 		error = PTR_ERR(file);
310 		goto err_fd;
311 	}
312 	file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
313 	file->f_flags |= O_LARGEFILE;
314 
315 	if (flags & MFD_ALLOW_SEALING) {
316 		file_seals = memfd_file_seals_ptr(file);
317 		*file_seals &= ~F_SEAL_SEAL;
318 	}
319 
320 	fd_install(fd, file);
321 	kfree(name);
322 	return fd;
323 
324 err_fd:
325 	put_unused_fd(fd);
326 err_name:
327 	kfree(name);
328 	return error;
329 }
330